Alex Holden, chief information security officer for Hold Security, which recently discovered more than a billion passwords had been compromised, said the breach may have started in late April or early May.

He and his colleagues said the same group of Russian and Ukrainian hackers suspected in the Target breach is behind the one at the Home Depot.

More than 40 million credit and debit cards were stolen late last year from Target stores.

Holden said the possible data breach at the Home Depot could be larger, and there's evidence the stolen information is already for sale on the black market.

"People should be taking a very close look at their credit card statement because if any fraudulent charges would appear to be on those cards -- maybe immediately or maybe over time," Holden said.

The Home Depot issued a statement Tuesday saying, "Protecting our customers' information is something we take extremely seriously, and we are aggressively gathering facts at this point while working to protect customers."

BACK TO HIS COMMUNITY. WE BEGIN WITH A DEVELOPING STORY, THAT MASSIVE SECURITY BREACH. IF YOU'VE SHOPPED AT HOME DEPOT RECENTLY, THERE'S A CHANCE YOUR CREDIT CARD INFORMATION HAS BEEN STOLEN. 12 NEWS' CHRISTINA PALLADINO IS LIVE IN WEST ALLIS. AND CHRISTINA, CYBER-SECURITY EXPERTS SAY THIS BREACH COULD BE BIGGER THAN THE ONE AT TARGET. WE HAVE LEARNED THAT POTENTIALLY EVERY HOME DEPOT IN THE UNITED STATES, INCLUDING THOSE ONE, COULD BE INCLUDED IN THIS HACK. WE TALKED TO ONE OF THE LEADING CYBER SECURITY TEAMS BASED IN THE MILWAUKEE AREA. THE CHIEF INFORMATION SECURITY OFFICER RECENTLY DISCOVERED MORE THAN ONE BILLION PASSWORDS HAD BEEN COMPROMISED, SAYS THE BREACH MAY HAVE STARTED IN LATE APRIL OR EARLY MAY. HE SAID THE SAME GROUP OF RUSSIAN AND UKRAINIAN HACKERS BEHIND THE TARGET BREACH ARE BEHIND US WHEN IT HOME DEPOT. MORE THAN 40 MILLION DEBIT AND CREDIT CARDS WERE STOLEN FROM TARGET STORES. THE POTENTIAL BREACH AT HOME DEPOT COULD BE LARGER. THERE IS EVIDENCE THAT THE INFORMATION IS ALREADY FOR SALE ON THE BLACK MARKET. PEOPLE SHOULD BE TAKING A CLOSER LOOK AT THEIR CREDIT CARD STATEMENTS. THE CHARGES MAY BE MADE IMMEDIATELY OR OVER TIME. WE REACHED OUT TO HOME TO THAT DEPOT, THEY DECLINED AN INTERVIEW. PROTECTING OUR CUSTOMERS INFORMATION IS SOMETHING WE TAKE EXTREMELY SERIOUSLY AND WE ARE GATHERING FACTS AT THIS POINT, WHILE WORKING TO PROTECT